Tunable optical instruments are a class of instruments in which a property of light, such as power level, is measured at more than one wavelength. Such instruments include optical spectrum analyzers (OSAs), optical channel monitors (OCMs) and others used in the wavelength division multiplexed (WDM) optical communication field, as well as other fields.
An OSA is a piece of equipment or a measurement instrument that receives an optical input and produces an output that represents the optical power contained in the optical input over a range of wavelengths. When we use the term “optical,” we mean to refer to a band of wavelengths of electromagnetic radiation including at least visible light, ultraviolet (UV) and infrared (IR). The tern should be viewed inclusively, particularly when an application of the described technology overlaps one of the explicitly named bands and also includes wavelengths marginally outside those bands.
Like all instruments realized in the “real world,” OSAs have a finite resolution determined by characteristics of the components of which they are constructed. However, spectrum analysis is generally thought of as producing a continuous representation of the power distribution across the measured band. That is, the power at each wavelength within the measured band is represented, even though resolution limits may mean that the represented power actually includes some power from nearby wavelengths, as well.
In some optical communication systems, plural channels of information may be transmitted on a single medium by modulating each channel onto a carrier having a different wavelength. Such systems include wavelength division multiplexed (WDM) systems. An instrument adapted to measure optical power at the specific, discrete channel carrier wavelengths is an OCM. Of particular, but not exclusive, interest to us are WDM systems operating with channel carrier wavelengths around 1500 nm.
Conventional OSAs and OCMs are bulky and expensive because the tunable filters used employ mechanical tuning means, such as moving a grating, stretching a fiber, etc. Such systems are also slow to tune, often requiring seconds to tune just a few nanometers in wavelength.